1. Technical Field
The present invention relates to a spark plug used for an internal combustion engine of a vehicle or the like.
2. Related Art
Conventionally, a spark plug is known as an ignition means for igniting air fuel mixture introduced into a combustion chamber of the internal combustion engine of a vehicle. For example, such a spark plug has a center electrode and an earth electrode with a spark discharge gap being interposed therebetween.
The lifetime of the spark plug depends on the increase of required voltage due to the extension of the spark discharge gap. That is, repeated spark discharge wears the center electrode and the earth electrode, thereby gradually extending the spark discharge gap therebetween. Accordingly, the required voltage increases which is required as voltage applied between the center electrode and the earth electrode for generating spark discharge. Then, the required voltage exceeds a predetermined value within a range in which some phenomena are not seen such as a decrease in dielectric resistance on a surface of insulator caused by, for example, smoldering. Hence, the spark plug reaches the lifetime thereof.
To solve this problem, various measures are considered for extending the lifetime of the spark plug by devising the structures of the center electrode and the earth electrode forming the spark discharge gap.
For example, JP-A-2007-250257 discloses a spark plug in which a plurality of earth electrodes are opposed to the center electrode to increase an opposing area (discharge area) between the earth electrodes and the center electrode, thereby restraining the extension of the spark discharge gap.
However, according to the measure for the spark plug disclosed in JP-A-2007-250257, in which the number of the earth electrodes opposed to the center electrode is increased, and the opposing area between the center electrode and the earth electrodes are increased, it may be difficult to easily increase the opposing area due to, for example, a constraint of the size of the center electrode or a housing holding the center electrode. Hence, measures are desired which can increase the opposing area between the center electrode and the earth electrodes by a simple structure without being influenced by the constraint of the size or the like.